Which type of microorganism is characterized as single-celled and lacking a nucleus?

The correct answer is bacteria, as they are defined as single-celled organisms that lack a nucleus. This characteristic places them in the category of prokaryotes, which are organisms whose cells do not have a membrane-bound nucleus. Instead, their genetic material is found in a single circular chromosome that is located in the cytoplasm.

Bacteria are crucial in various ecological processes, including decomposition and nutrient cycling, and they are also integral to human health, both as pathogens and in beneficial roles like gut flora. Their relatively simple structure enables rapid reproduction and adaptation to various environments.

In contrast, fungi are multicellular (or in some cases, unicellular, such as yeasts) and have a complex cell structure with a nucleus. Protozoa are also single-celled organisms but are eukaryotic, meaning they do have a nucleus. Viruses, while they can infect bacteria and other cells, are not classified as living organisms and lack cellular structures entirely—they consist of a protein coat and genetic material (either DNA or RNA) but cannot reproduce independently outside of a host organism. Thus, bacteria stands out as the correct answer due to its defining characteristics.